Valves and their miniature counterparts, microvalves, control the flow of fluids (e.g., gas or liquids) in fluidic systems. Microvalves have generally improved fluid flow control in applications where the significant power demands of a macro-scale valve would be unsuitable. Microvalves also generally avoid large dead volumes—the undesirable empty space in a valving or other fluidic device that must be pressurized when flow starts and depressurized when flow stops. Microvalves having a small dead volume present faster response times than macro-scale valves. Notwithstanding these improvements from the macro-scale regime, past microvalve designs have left room for improvement in both power consumption and response time, as well as in connection with other valve performance parameters and fabrication considerations.
A number of different microvalve designs and actuation schemes have been introduced. Electromagnetic microactuators have been demonstrated, although magnetic forces scale unfavorably for devices with small volume. Piezoelectric actuators have shown substantial actuation force and fast response times, but have also required large operating voltages and a complex, stacked hybrid construction in order to achieve substantial actuation displacement.
Many commercially available microvalves have relied upon some type of thermal actuation. Unfortunately, shape memory alloy (SMA) and bimetallic thermal actuators (i.e., bimorph structures) have tended to require significant power for actuation, typically hundreds to thousands of milliwatts.
Thermopneumatic microvalves have also been reported, but the typical overall power consumption of such valves has also been undesirably high.
In contrast, microvalves utilizing electrostatic actuation schemes have shown near-zero power consumption. However, such microvalves have been susceptible to particulate contamination and weak actuation force over large distances. Electrostatically actuated microvalves have been unsuitable for applications requiring long valve throw (i.e., large valve travel distances) and accordingly been designed with low-flow rate applications in mind. See, for example, Robertson et al., “A Nested Electrostatically-Actuated Microvalve for an Integrated Microflow Controller,” MEMS 1994 Proceedings, IEEE Workshop, pp. 7–12 (1994).
Past microvalves have minimized power consumption through bistable designs, where power is required only during switching. A bistable microvalve taught by Wagner et al. relies upon electrostatic actuation to drive a pair of buckled membranes acted upon pneumatically via a pair of linked cavities. When one membrane is pulled down electrostatically, the other membrane is pushed up pneumatically. Wagner et al., “Micromachined Bistable Valves for Implantable Drug Delivery Systems,” 18th Annual International Conference of the IEEE Engineering in Medicine and Biology Society, pp 254–255 (1996). However, such approaches to bistable valves are burdened by complexity in both design and fabrication process, and are also unsuitable for high pressure applications.